The material used in medical situations can consist of widely different groups of materials such as metals, polymers or ceramics. In such situations, materials are sought which are stable from the point of view of corrosion and erosion, and which have good strength characteristics in vivo. In addition to high mechanical and chemical stability, the material must exhibit good biocompatibility. For implants which are intended to support loads, it is also of great importance for the material to have good bone-anchoring ability by means of the contact zone between newly formed bone and implant exhibiting high shearing forces, i.e. high binding strength between implant and bone. Examples of materials with low load-bearing ability but with good anchoring in surrounding tissue are hydroxyapatite and fluoroapatite. In the presence of hydroxyapatite or biologically active glass (calcium phosphate-containing material) in the implant material, direct contact has been reported between bone tissue and implant at the atomic level (inter alia, Tracy and Doremus, J. Biomed. Mater. Res. Vol. 18, 719-726 (1984), and Hench, J. Am. Ceram. Soc., Vol. 74, page 1501 (1991)). However, a pure apatite phase or high content of apatite (in excess of 50% by volume) affords too little strength and a material which is inclined towards slow fissure growth (e.g. Metal and Ceramic Biomaterials, Vol. II, FIG. 5, page 52, CRC Press, 1982).
A combination of the aspects of strength and good anchoring ability is described in the patent literature by McGee, U.S. Pat. No. 3,787,900 (1974). The material here is a composite of spinel and Ca phosphate. However, the strength of such materials is relatively low. Combining increased strength and anchoring ability has previously been demonstrated by mixing apatite and a construction oxide, for example zirconium oxide, in a prescribed manner (Swedish Patent 465 571) and by densifying by means of hot isostatic pressing. In these materials, which are in the form of bulk materials--not surface layers as in the present invention--the Ca phosphate phase content is limited so as not to adversely affect the strength, as is explained by Experiment 2 in the text and Patent claim 5 in said Swedish Patent 465 571, where the Ca phosphate content is limited to 5-35 % by volume, preferably 10-25% by volume. This accords with general strength development upon introduction of a weaker phase, where the strength falls dramatically at contents in excess of about 20% by volume. For the zirconium oxide/hydroxyapatite system, the dependence of the strength on the apatite content is described in detail by Li et al. (Biomaterials Vol. 17, page 1789, FIG. 2, 1996).
A surface layer of Ca phosphate is an established technique for implants. A number of different methods for applying such surface layers are reported in the literature, such as wet chemical methods (e.g. sol-gel methods) with subsequent sintering, electrode deposition methods, plasma spraying and pulsed laser deposition, gas deposition methods (CVD and PVD) and hot isostatic pressing. Plasma spraying is generally used as the coating method. Use of hot isostatic pressing is reported in the literature for pure hydroxyapatite with layer thicknesses of 20-50 micrometres, i.e. considerably thicker than is envisaged in the present invention, and with problems in obtaining sufficient quality in terms of the micro-structure and strength of the surface layers, see Heide and Roth, Int. Conf. on Hot Isostatic Pressing, June 1987, and Hero et al., J. Biomed. Mater. Res., Vol. 28, 343-348, 1994.
With a biocompatible phase limited to a very thin surface layer having a specific maximum thickness calculated from the basic equation governing the fracture mechanics, it is possible, according to the invention, to increase the strength of the surface layer in such a way that this does not limit the strength of the component. This means that the optimum core can be chosen from the point of view of general design function, e.g. in terms of strength and in terms of machining (shaping), and that the optimum surface layer can be chosen from the point of view of biomaterials, i.e. biocompatible and with good anchoring ability. This makes it possible to maximize both the strength of the implant element and its anchoring in the surrounding tissue.
The aspects relating to fracture mechanics are of central importance to the present invention, and for these reference is made generally to Handbook 6, edition 3, "Pulverteknik", chapter 7 CERAMICS, in particular pages 7-24 to 7-39, published by MMS (now SMS), 1995.
Other related aspects of the invention, especially with regard to biological response, are treated in the scientific literature and also, inter alia, in the following patent publications.
DE 3301122--sintering of titanium dioxide and hydroxy-apatite at high temperatures where disintegration of hydroxyapatite takes place; U.S. Pat. No. 4,149,893--hot pressing in an open system of pure hydroxyapatite; U.S. Pat. No. 4 ,599,085 sintering and extrusion of metal with macroscopic hydroxyapatite areas--of the order of 100-500 micrometres; and EP 0 328 041--porous layer of zirconium oxide and Ca phosphate on a sintered body of zirconium oxide.
The present invention concerns high-strength, fatigue-resistant materials with maximum anchoring ability, produced by a processing technique which simplifies and at the same time extends the scope of application of surface layers, which have a favourable biological response, to different types of core materials independently of the geometry of the core material.